This invention relates generally to video signal processing and more particularly to improved methods of peaking signals, especially coded digital video signals.
Peaking (signal enhancement) is commonly applied to an output video signal before the signal is displayed, in order to enhance high frequency edges to enhance the sharpness of a displayed image. A conventional video signal is generally comprised of luminance and color components. A luminance signal (or component of a signal) is frequently designated a Y signal or signal component and a chrominance signal (or component of a signal) is frequently designated a P signal or signal component. Video signals generally contain vertical, horizontal, diagonal and temporal details. Although it is possible to apply peaking in any of these directions (e.g. vertical or horizontal), such signal enhancement is typically only performed in the horizontal direction.
Conventional peaking systems commonly employ a filter, such as a high pass filter, to select the high frequency portions of a signal. Typical television sharpness controls apply a horizontal filter to the luminance signal. The output of the high pass filter is then added to the source video signal. If the source video signal contains high frequency components, these components will be output from the high pass filter and may be adjusted in amplitude based on the amount of peaking desired, and when added back to the source video signal, the high frequency components of the source video signal will be boosted (increased) in amplitude. The net effect is to xe2x80x9cpeakxe2x80x9d or xe2x80x9csharpenxe2x80x9d the image by increasing the slope of the signal corresponding to edges of the displayed image. Thus, the displayed image, after peaking, will look sharper or crisper. The inverse may also be applied to decrease the high frequency components and actually xe2x80x9csoftenxe2x80x9d the image.
Image signals are commonly encoded digitally for transmission and then decoded for display. For example, the Motion Pictures Experts Group is an ISO group that sets standards (MPEG standards) for compressing and storing video, audio and animation in digital form. MPEG-1 is a standard audio and video coding format for low density storage media such as CD-ROMs video CDs and so forth. MPEG-2 is a standard coding format for broadcast video. In MPEG, the compression method involves I-frames or intra frames. A frame refers to a complete TV picture. It can be made up of multiple fields, such as a field of odd numbered lines and a field of even-numbered lines. An I-frame is an intra-coded video frame that is independent of other video frames in MPEG standard. I-frame is repeated at a regular interval to refresh the coding sequence. Between any two I-frames are P-frames (Predictive frames) and B-frames (Bi-directional predictive frames). The P- and B-frames only contain information or changes between the I-frames. An I-frame and its P- and B-frames are called a GOP (Group of Pictures).
MPEG compression methods are lossy compression methods. When color images are coded, decoded, and subjected to digital post processing, the display can develop undeliverable characteristics. For example, when a portion of an image has saturated colors, that portion of the image can experience significant enhancement of noise when conventional filters are applied to peak the image. This can be particularly true of blue sections of an image, to which the eye is most sensitive. Thus, while sharpening edges, conventional peaking systems and/or filters can add objectionable noise to regions intended to lack color details.
Video signals typically involve some type of noise component. The noise can be present in the actual video signal. Noise can also be introduced by processing circuitry, such as at the encoder, decoder, transmitter, analog-digital converter (ADC), digital-analog converter (DAC) and so forth. Because the frequency of the noise will often pass through the high pass filter, the noise envelope of the signal can become amplified by a conventional peaking filter. A portion of a video signal might be intended to be flat (i.e., containing no details). Increasing the noise envelope in such a portion of a signal can be particularly subjectively noticeable and troublesome when it occurs in a portion of a video signal intended to display a uniform monochrome area.
Various methods have been proposed for improving conventional video peaking systems. For example, U.S. Pat. No. 4,466,016 discloses television signal filtering systems and systems for separating television luminance and chrominance signals for subsequent video information processing. U.S. Pat. No. 5,416,532 describes adaptive video peaking circuitry and systems for use with a luminance/chrominance separator. The reference describes circuits and systems for analyzing nine pixel arrays, applying various filters and constructively combining horizontal and vertical peaking signals. Various other peaking and noise coring proposals are disclosed in U.S. Pat. No. 5,124,794, U.S. Pat. No. 5,161,015, U.S. Pat. Nos. 4,597,011, 4,635,119, 5,012,329 and WO 89/11196. The contents of all of these are incorporated herein by reference. These proposals are not believed to be fully satisfactory, as for example, being only applicable to analog rather than digital signals, not being satisfactory or applicable to signals in accordance with MPEG or ATSC standards or unsatisfactorily increasing noise in undesirable locations of an image.
Accordingly, it is desirable to provide improved methods and systems for peaking a video signal and overcoming drawbacks of the prior art.
Generally speaking, in accordance with the invention, a method and apparatus for the selective peaking of portions of a video signal is provided. The video signals, such as those transmitted in compressed form in accordance with MPEG Standards, can be presented as blocks of data encoded using Discrete Cosine Transforms (DCT). The Discrete Cosine Transform coefficients of a signal can be readily obtained and analyzed. If the analysis of the DCT coefficients detects appropriate energy levels, such as those characteristic of edge portions of an image or other details in a portion of an image, that portion of the signal can be peaked. Portions of a signal corresponding to flat images can be minimally peaked, or not peaked at all. The analysis of the signal and corresponding amounts of enhancement (peaking), if any, can be performed on a block-by-block basis to selectively peak portions of a video signal. Thus, if the DCT coefficients for a portion of the signal exceed or fall within a pre-selected value or range, that portion of the signal can be subjected to peaking. For example, if the amplitude of the high frequency horizontal frequency DCT coefficients exceed (or fall within) a pre-set threshold value or range, a horizontal peaking circuit can be set to peak that portion of the signal. Peaking can also be performed in the vertical direction or both horizontally and vertically. The signal processor controlling such peaking can be set for gain in proportion to how much the DCT coefficients exceed the threshold.
An analog signal can be sent in parallel to an analog-to-digital converter and then to a processor which obtains the DCT coefficients of the converted signal. These coefficients can be analyzed and used to control the adaptive peaking filter coefficients for the spatial block corresponding to the given DCT block for the corresponding portion of the analog signal. The analog signal can also be converted to a digital signal, peaked in accordance with the invention and converted back to an analog signal.
In a preferred embodiment of the invention, the peaking of the luminance, chrominance or both portions of the signal can be selectively controlled based on the spectral histogram of the chrominance energy and/or chrominance difference signals present in the image signal. Such chrominance energy can be ascertained by analyzing the DCT coefficients of an MPEG encoded signal.
Accordingly, it is desirable to provide an improved system and method for peaking a video signal and an improved peaked signal resulting from such system and the display of such signal.
Another object of the invention is to provide an improved peaked signal resulting from such system and the display of such signal.
Another object of the invention is to provide an improved system and method for peaking a compressed digital video signal.
Yet another object of the invention is to provide an improved signal processor for peaking a video signal.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the product and signal adapted to effect or resulting from such steps, all as exemplified in the following detailed description and drawings and the scope of the invention will be indicated in the claims.